[feat] extend NamedMultiReservable

[stanly-johnson]

Changelog:

• Adds ensure_reserved_named, unreserve_all_named, slash_all_reserved_named, repatriate_all_reserved_named functions to NamedMultiReservable Currency.

Resolves #731

[codecov-commenter]

Codecov Report

Merging #732 (e7d87e9) into master (67f4a79) will increase coverage by 0.14%.
The diff coverage is 94.02%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
+ Coverage   77.52%   77.67%   +0.14%     
==========================================
  Files          88       88              
  Lines        9003     9070      +67     
==========================================
+ Hits         6980     7045      +65     
- Misses       2023     2025       +2     

Impacted Files	Coverage Δ
traits/src/currency.rs	60.00% <78.94%> (+60.00%)	⬆️
tokens/src/tests.rs	99.87% <100.00%> (+<0.01%)	⬆️
tokens/src/lib.rs	83.89% <0.00%> (+0.25%)	⬆️

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